Test DMing a po-box if we don't do that already

[Powersource]

Followup from #96 (comment)

Also maybe related #96 (comment) #96 (comment)

• look for an existing test
• add a test

[Powersource]

looks like it's tested in test/unbox.test.js

it loops over

  const RECPS = [
    ssb.id,
    groupId,
    poBoxId
  ]

and tries to publish and read one message for each